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Republican Sen. Ron Johnson promised that, if re-elected, he could be a check on a President Clinton

It was a preview of the playbook that Republicans could employ

Green Bay, Wisconsin (CNN)Sen. Ron Johnson, suddenly surging in a re-election race where had been assumed all but dead by national Republicans, offered Friday what could be a closing message for Republicans nationwide: I am a check on Hillary Clinton.

Ever since Donald Trump began plummeting in nationwide and swing-state polls, Republicans in tough re-election fights have been weighing how aggressively to elbow out their presidential candidate. Several competitive candidates have disavowed Trump entirely -- but others have tried to traipse the tightrope.

Debating here in the shadow of Lambeau Field in his first showdown against his Democratic challenger, former Sen. Russ Feingold, Johnson attempted to do just that, rolling out a check-and-balance argument.

The strategy allows Johnson -- and other Republicans -- to effectively concede the presidential race without having to invite Trump's scorn with an un-endorsement.

"I'm not going to defend the indefensible," said Johnson when asked if he could pledge to "100%" support Trump given the spate of recent allegations of sexual misconduct against him. "But I will hold whoever is our president accountable."

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It was a preview of the playbook that Republicans could employ in the final three weeks before Election Day -- broadly and non-controversially defending Trump, relentlessly attacking Clinton and promising to not let Clinton steamroll him if both are sent to Washington.

Yet Johnson left open the door to abandoning Trump should he look more and more poisonous to down-ballot candidates. Johnson, speaking to media after the debate wrapped, first dismissed a question about ditching Trump if he is getting "destroyed" as a "hypothetical" -- but then left open the possibility when pressed by a reporter.

"Sure, anybody could do something that would make me withdraw my support," Johnson said. "Donald Trump's a change agent. So am I."

Johnson had been long thought to be one of the two Republican incumbents almost certain to be replaced by a Democratic challenger. Despite being an incumbent, Johnson is actually lesser well-known than Feingold, the state's longtime Demcoratic senator who Johnson ousted in 2010.

But after leading Johnson for nearly the entire year by double digits, the race has surprisingly become competitive: the most recent survey, by Marquette University Law School, showed Feingold leading by just 2 points.

That has invited some increased scrutiny of Feingold, who on Friday did not prosecute the anti-Trump campaign that Senate Democratic candidates are executing nationwide. After the first question about Johnson's fealty to his nominee, Feingold did not utter "Trump" a single other time during the entire hour-long debate.

Johnson similarly tried to place Clinton's unpopularity as a weight on his Democratic opponent, and by the end of the evening, both candidates were calling for their rivals to renounce their party's nominee.

Feingold, speaking to reporters after the debate, was a late endorser of Clnton -- even on Friday, he wouldn't say whether he supported fellow liberal Bernie Sanders in the primary -- and he labored to tell reporters that Clinton would carry out an "ethical presidency."